Driving Coast to Coast across america

I'm thinking about a trip next summer with a mate which involves
driving from Boston to San Fransisco. Ideally, we'd like to stay on the
east coast and visit florida to see the Kennedy space centre. from
there through texas and across to Las vegas. The kind of time scale we
were thinking of is about 2 months or there abouts. Does anyone have
any suggestions on how to go about this in terms of places to stay each
night (camp, motel...) and also in terms of transportation. Renting a
car could get pricey and seeing as we're 21 and from the UK then there
might be issues with age and driving licenses. Would it be worth buying
an old rust bucket and then selling it at the other end??? any
information on anything would be great,

thanks

Oli Sander